Coal liquefaction solids removal

ABSTRACT

SOLID RESIDUES ARE MORE EFFECTIVELY SEPARATED FROM A COAL EXTRACT ENRICHED SOLVENT OF A COAL LIQUEFACTION PRODUCT, IN A SOLIDS-LIQUIDS SEPARATION ZONE IN WHICH SOLIDS SIZE IS A SEPARTION PARAMETER, BY ADDING TO THE COAL LIQUEFACTION PRODUCT A COAL EXTRACT LIQUID DERIVED FROM THE COAL LIQUEFACTION PRODUCT AND CONTAINING AT LEAST 20 VOLUME PERCENT OF MATERIALS BOILING BELOW ABOUT 400* F. OR AT LEAST 20 VOLUME PERCENT OF MATERIALS BOILING ABOVE ABOUT 1000* F.

Feb. 5, 1974 R. J. FIOCCO ETAL COAL LI'QUEFACTION SOLIDS REMOVAL 3Sheets-Sheet 2 Filed April 24, 1972 SIZE OF SOLIDS MICRONS o WU m P V wM w w mi w m m M FIG. 2.

Feb. 5, 1974 R. J. FIOCCO ETAL 3,790,467

COAL LIQUEFACTION SOLIDS REMOVAL Filed April 24, 1972 3 Sheets-Sheet 5All/\VUS ouloads 3.LVUlN3O Ell Illll llnl IIII

United States Patent 3,790,46 COAL LIQUEFACTION SOLIDS REMOVAL Robert J.Fiocco, Summit, NJ., and Edward L. Wilson,

Baytown, Tex., assignors to Esso Research and Engineering CompanyContinuation-impart of application Ser. No. 67,457, Aug. 27, 1970, nowPatent No. 3,687,837. This application Apr. 24, 1972, Ser. No. 246,725The portion of the term of the patent subsequent to Jan. 29, 1989, hasbeen disclaimed Int. Cl. Cg 1/04 US. Cl. 208-8 12 Claims ABSTRACT OF THEDISCLOSURE Solid residues are more effectively separated from a coalextract enriched solvent of a coal liquefaction product, in asolids-liquids separation zone in which solids size is a separationparameter, by adding to the coal liquefaction product a coal extractliquid derived from the coal liquefaction product and containing atleast 20 volume percent of materials boiling below about 400 F. or atleast 20 volume percent of materials boiling above about 1000 F.

CROSS REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of application Ser. No. 67,457, filed Aug. 27, 1970and entitled Coal Liquefaction Solids Removal, now US. Pat. 3,687,837.

BACKGROUND OF THE INVENTION This invention relates to the production ofliquid fuels from solid coal, and more particularly, to a step in suchprocesses in which undissolved solids in a coal extract enriched solventof a coal liquefaction product are separated from the enriched solventaccording to the size of the solids.

In the upgrading of coal by hydrogen transfer and cracking to obtainliquid fuels such as gasoline and kero sene, a number of processes arecarried out to transfer hydrogen to the basic coal molecules, to breakup the coal molecules into smaller fragments, and to remove sulfur andnitrogen from the products. The basic coal liquefaction step may becarried out in a number of ways, but it is preferred to use ahydrogen-donor solvent, such as hydrogenated creosote oil or anindigenous hydrogenated product boiling in a middle distillate rangewhich is obtained in the liquefaction of coal. Extraneous molecularhydrogen may be added to the liquefaction zone, ifdesired. All of this,directed to the basic liquefaction of coal, is old in the art, asdisclosed in various patents such as US. Pats. 3,018,241 and 3,117,921.

The product of coal liquefaction is a mixture of liquefied coal extract(some of which has been cracked and hydrogenated), solvent, andundissolved solids, including unconverted organic solids and ash solids.The solids are conventionally separated from the liquefied extract andsolvent by centrifugation, filtration, or other solids-liquid separationprocesses in which solids are separated from liquids according to thesize of the solids. Thereafter the clarified extract enriched solvent isupgraded by various processes, typically including catalytichydrocracking, in order to produce liquid fuel products.

In processes in which the clarified extract enriched solvent from thesolids-liquid separation zone is passed into a catalytic hydrocrackerfor upgrading, extremely small solids (for example, 10 microns and less)remaining in the clarified liquid can block catalyst pores andeventually produce channeling in the bed. It is important that theseextremely small particles be removed from the liquid extract beforehydrocracking of the extract.

Heretofore, in US. Pat. 3,018,241, it was suggested that benzeneinsoluble solids can be separated from the extract enriched solvent bythe addition of a low-boiling paraffinic antisolvent, specificallyhexane. On a commercial scale, however, the suggestion is impractical.Addition of such antisolvents precipitates considerable amounts of thebenzene soluble coal extract liquid gained by coal liquefaction,decreasing the yields of liquid extract to the extent that, as a methodof improving solids removal from the coal liquefaction product, use ofsuch antisolvents is economically prohibitive. It is therefore highlydesirable and plainly important that more economical and more usefullyeffective measures be discovered for improving the removal of solidsfrom coal extracts produced in coal liquefaction processes. Thisinvention is directed to that end.

SUMMARY OF THE INVENTION This invention provides a new method forimproving the removal of solids, particularly very small solids on theorder of 10 microns and less, from the coal extract enriched solvent ofa coal liquefaction product, while preserving the yields of coal extractobtained in a coal liquefaction process.

According to the invention, a coal extract liquid derived from the coalliquefaction product and containing at least about 20 volume percent ofmaterials boiling below about 400 F. or at least about 20 volume percentof materials boiling above about 1000 F. is added to the coalliquefaction product in an amount which is effective to make at least aportion of the smaller and difficultly separable solids in the coalliquefaction product into solids that are larger and more easilyseparable in a solids-liquids separation zone wherein solids size is aseparation parameter.

Coal derived materials boiling below about 400 F. or above about 1000 F.are so sufficiently dissimilar to the coal extract enriched solvent thatthey are able to cast from solution or resolidify small amounts ofquasisolid coal extract materials liquefied in a deep extraction of thecoal; however, the coal derived materials boiling below about 400 F. orabove about 1000 F. are not so dissimilar from the extract enrichedsolvent that they noticeably cause other liquefied coal extractmaterials to be cast from solution. It has been found that the smallamounts of quasi-solid materials so cast from solution are effective tocause a sufficient increase in size of smaller, difiicultly separablesolids that such solids are made more separable from the extractenriched solvent in a solidsliquids separation zone such as acentrifuge, filter, cyclone or other type of such zone in which solidssize is a parameter of separation. Because only small amounts, typicallyless than about 5 weight percent of the quasi-solid materials are castfrom solution, and also because substantially no other coal extractmaterials than the quasisolids are so cast from solution, the yield ofcoal extract materials obtained by the coal liquefaction operation ispreserved.

The coal derived liquid which is added to the coal liquefaction productto improve solids removal in a solidsliquids separation zone preferablyis obtained from the clarified extract enriched solvent, before or aftera hydrocracking operation conducted on such solid, or in the case of acoal extract liquid derived from the coal liquefaction product andcontaining about volume percent of materials boiling below about 400 F.,from a fraction a coker oil stream produced by coking the slurry ofsolid residues obtained from the solid-liquid separating zone.

In general, whichever fraction is used, suitable increases in coalextract clarity can be obtained by adding the fraction of coal derivedliquid to the liquefaction product in amounts of from about 1 to about50 weight percent of the resulting feed to the separation zone.

As an aspect, it has been found that a desired clarity of the coalextract issuing from the separation zone in a continuous process can bemaintained by adding to the liquefaction product more or less of afraction of a coal derived liquid which contains at least 20 volumepercent of materials boiling below about 400 F., as needed to keep thespecific gravity of the clarified extract below a predetermined value inthe range from about 1.08 to about 1.12 above which a desired level ofclarity is not obtained.

Other aspects and advantages of the invention will be more evident froma description of preferred methods of carrying out the invention, takenin conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flow diagram ofpreferred modes of carrying out the invention;

FIG. 2 is a plot of the weight percent of solids smaller than a givensize versus the particle sizes of pyridinewashed solids and methyl ethylketone-washed solids of a liquefaction product prepared as described inExample 1, where the plot is discussed; and

FIG. 3 is a plot of the ash content and specific gravity of the overflowconcentrate from a centrifuge operated in accordance with this inventionover a 140-hour period, indicating the rate of addition of a recyclecoker oil fraction derived from the centrifuge underfiow. FIG. 3 isdiscussed in Example 2.

comprises the liquefied coal extract, at least a partially DESCRIPTIONOF THE PREFERRED EMBODIMENT Referring to the FIG. 1, raw coal is fed byway of line 10 into a mixer 11 which also receives by line 10 a hydrogendonor solvent, preferably an indigenous hydrogenated product boiling ina middle distillate range, suitably recycled from line 47. The coal isslurried in the solvent oil, and the slurry is withdrawn by way of line12. Any suitable coal-like material may be used as a feedstock, forexample, subbituminous coal, bituminous coal, lignite or brown coal. Thecoal is generally ground to a particle size of about -8 mesh (Tylerscreen) and finer and may be dried before it is fed into the mixer 11.The solvent/coal weight ratio is suitably within the range from about0.8 to about 10, and preferably from about 1 to about 2.

The slurry may be mixed with hydrogen introduced by way of line 13 andthen passed into a liquefaction reactor 14. Within the liquefactionreactor, the coal is allowed to dissolve under conditions of hightemperature and pressure, such as a temperature within the range fromabout 650 F. to about 900 F. and a pressure from about 350 p.s.i.g. toabout 2500 p.s.i.g. The hydrogen-treat rate (if hydrogen is used) may befairly low, and may suitably range from about 100 to about 1000s.c.f./b. of coal slurry charge. In the liquefaction reactor, the coalis depolymerized and partially thermally cracked, and a liquefactionproduct is Withdrawn, by way of line 15, which including ash solids. Thehydrogen and noncondensable gases are separated from the liquid andsolid components in a separator 16 and are removed by way of line 17,while the slurry is carried by line 18 to a separation zone 19 whichseparates solids from liquids according to sizes, separately issuing aclarified coal extract and a concentrated solid slurry. Separation zone19 is suitably a filter, a hydrocyclone, or, as illustrated, acentrifuge, in which case the clarified coal extract issues as aconcentrate overflow 20 and the concentrated solids slurry issues as anunderflow 21.

In accordance with this invention, there is added to the coalliquefaction product feeding to centrifuge 19, a fraction of a coalextract liquid containing at least about 20 volume percent of materialsboiling below 400 F. or above 1000 F., suitably a clarified coal extractfraction boiling within the range of from about F. to about 700 F .orabove 1000" F. and containing ash solids, or a distillate fraction ofboiling in the range of from 100 F. to 700 F. obtained from productsderived from coking the concentrated solids slurry from separation zone19.

The added fraction of clarified coal extract having 20 volume percent ofmaterial boiling below 400 F. may be obtained from the clarified coalextract without subjecting the extract to further treatment after itissues from the separation zone. Thus, the clarified coal extract may befractionated, as in a pressure reduction flash zone fractionationhereinafter described, to recover such fraction which then is recycledfor addition to the liquefaction product. Alternatively, the clarifiedcoal extract may be further treated to upgrade the extract. For example,the clarified coal extract may be hydrotreated to saturate theunsaturated mono and polynuclear rings of the ex tract to a desiredlevel; or in a more severe treatment, the clarified coal extract may behydrocracked to convert high-boiling aromatic compounds to lower boilingand saturated compounds of napthenic character. The use of ahydrocracking process is hereinafter described to illustrate this aspectof the invention. The desired fraction can be recovered from thehydrotreated or hydrocracked effluent and recycled for addition to thefeed to the centrifuge.

In the drawing, reference numeral 22 indicates a recycle line ofclarified coal extract boiling within the range of from about 100 F. toabout 700 F. and containing at least about 20 volume percent ofmaterials boiling below about 400 F.; reference numeral 23 indicates arecycle line of coker oil distillate fractions boiling within the 100 F.to 700 F. range, having at least 20 volume percent of materials boilingbelow about 400 F. and recovered from the c'oked concentrated solidsslurry underflow from centrifuge 19; and reference numeral 24 indicatesa recycle line of a clarified coal extract fraction boiling above about1000" F. and containing ash solids.

Whichever of the fractions from lines 22, 23, and 24- is used,sufiicient of the fraction is added to produce an increase inclarification of the clarified overflow centrate issued from thecentrifuge by line 20. Suitably, the fraction added by line 22, line 23,or line 24 is in amounts of from about 1 to about 50 Weight percent,preferably from about 3 to about 30 weight percent of the resulting feedto the centrifuge. Additions of more than about 50 weight percent of arecycle fraction are impractical from a cost standpoint.

When the recycle fractions boiling within the 100 F. to 700? F. rangeare added, the specific gravity of the clarified extract is suitablymonitored. A rise in specific gravity above a predetermined level in therange of about 1.08 to 1.12 corresponding to a desired centrate clarityis followed by addition of more of the recycle fraction, as needed, toreduce the specific gravity of the centrate to the predetermined level.

The clarified centrate overflow from centrifuge 19, as stated above, maybe fractionated without further treatment, or it may be subjected tofurther upgrading and then fractionated. Accordingly, in the formerinstance, the overflow centrate from centrifuge 19 is carried by line 25from line 20 with the opening of valve 26 and the closure of valve 27into a pressure reduction flash zone 28, which is operated in accordancewith known technology to recover a fraction of the centrate overflowwhich boils within the range of from about 100 F. to about 700 F. andcontaining at least about 20 volume percent of materials boiling belowabout 400 F. The particular fraction selected within that range isdischarged from the flash zone by way of line 29 and passes through ametering valve 30 which admits only as much of that fraction to recycleline 22 as is desired, the remaining portion of that fraction beingdiverted into line 31 for recycle to line 20 by way of line 32.Fractions of the centrate fed to the flash zone which are not wished tobe recycled, i.e., fractions boiling above about 700 F. but below about1000 F., or in a particular instance, a heavier fraction within the 100F. to 700 F. boiling range aforesaid, are discharged from flash zone 28by line 33 for return to line 20 by line 32.

Flash zone 28 may be operated to separate the foregoing fractions fromthe fraction of the overflow centrate boiling above about 1000 R, whichis illustrated as discharged from flash zone 28 by way of line 34,which, when valve 35 is opened and valve 36 is closed, passes thebottoms fraction of the centrate by way of line 37 into the centratebottoms recycle line 24. When valve 35 is closed and valve 36 is opened,line 34 returns the centrate bottoms to line 20 for upgrading.

Line 20 carries clarified centrate overflow into a hydrocracking zone,suitably comprising two reactors 38 and 39, for upgrading. The clarifiedcentrate in line 20 may comprise all of the centrate overflow fromcentrifuge 19 in the instance when valve 26 is closed and valve 27 isopened, or various fractions of the centrate overflow return to line 20by lines 36 and/or 32 when valve 27 is closed and valve 26 is opened.

In the hydrocracking zone, the clarified extract is contacted withhydrogen introduced by way of lines 40 and 41 and is passed sequentiallyby way of line 42 in downflow across stationary beds of catalystgranules in the reactor, suitably cobalt molybdate, nickel molybdate,nickel tungsten and palladium on various substrates, such askielselguhr, alumina, silica, faujasites, etc. The cobalt molybdatecatalyst is preferred, and may have 3.4 weight percent cobalt oxide,12.8 weight percent molybdenum oxide, and 8.3 weight percent alumina.The catalyst may range from 2 to 5 weight percent cobalt oxide and fromto weight percent molybdenum oxide, all as well known in hydrocrackingarts.

The clarified extract passed in downflow across the catalyst beds ispreferably in the liquid phase, but may be in the mixed liquid and vaporphase, hydrogen in the reactor being present in both the gas phase anddissolved in the liquid phase. Preferably, the hydrocracking reactioncarried out in the reactors 23 and 24 occurs under hydrocrackingconditions which include a temperature from about 650 F. to about 900 F.preferably about 750 F., a pressure of 1000 to 4000 p.s.i.g., preferably2000 p.s.i.g., a residence time within the reactor of 30 to 300 minutes,preferably 60 minutes, and a hydrogen rate from about 3000 to about 8000s.c.f./b., preferably about 5000 s.c.f./b., based on the total volume ofliquid charged to the hydrocracking reactors.

The products of the hydrocracking reactor are removed by way of line 43and introduced into a fractionating tower 44, where the clarifiedhydrocracked products are fractionated into a plurality of various fuelproducts streams, including: gas taken overhead by way of line 45; astream boiling within the naphtha boiling range, from about 75 F. to 100F. up to about 400 F., a portion of which is removed by way of line 46for recycle by way of line 22; a middle distillates stream boiling overthe range from about 400 F. to about 700 F., a fraction of which may beremoved by way of line 47 for introduction to recycle line 22; a heavydistillates stream boiling above about 700 F. and up to about 1000 R,which is removed by Way of line 48 for use as desired; and a clarifiedcentrate fraction boiling above about 1000 F. and containing ash solids,which is removed from distillation tower 44 by way of line 49 forrecycle to line 24.

The. fraction taken from the distillation tower 44 by way of line 47 maybe introduced alone into line 22, or blended so as to have at leastabout 20 volume percent of materials boiling below about 400 F., by theclosure or metering operation of a suitable valve 50.

The bottoms fraction boiling above about 1000 F. taken from distillationtower 44 by line 49 is recycled to line 20 for cracking by way of line51 when valves 52 and 53 are closed and valve 54 is opened. The centratebottoms fraction is recycled to line 24 for addition to the centrifugefeed by line 49 on closure of valves 53 and 54 and the opening of valve52. When valves 54 and 52 are closed and valve 53 is opened, the bottomsfraction of the centrate is carried by way of line 55 to a mixing zone56 where it is mixed with the underflow carried by line 21 in the mixingzone 56. Line 55 may be closed by valve 53 to prevent mixing of theunderflow from line 21 and the bottoms fraction from distillation tower44, if desired.

From mixing zone 56, effluent discharges by way of line 57 forintroduction into coker 58, which preferably is operated to maintain adense phase fluidized bed of coke particles in the lower portionthereof. Within coker 58, the liquid hydrocarbons in the underflowundergo thermal cracking, and vaporous hydrocarbon products are passedupwardly into a distillation tower suitably mounted above the cokervessel as schematically illustrated. The fractionator is operated toproduce gas, naphtha, middle distillates and heavy distillates streams,as in the case of distillation tower 44, the desired fractions withinthe naphtha and middle distillates streams being removed to recycle line23 by way of lines 59 and 60 respectively. The use of single portions ofsuch fractions or blends thereof so as to recycle a stream having atleast 20 volume percent of materials boiling below about 400 F. iscontrolled by a valve 61, as in the case of valve 50 for the fractionsproduced from distillation tower 44. The molecules in the fractionrecycled from the coker will be more aromatic with less hydrogencontent, than the molecules in the fraction recovered from tower 44, andmore materials boiling below 400 F. will in general be necessary toproduce a desired improvement in the clarity of the centrifuge overflow.

Although not illustrated, the vaporous product from coker 58 may berouted directly to distillation tower 44 and recovered therefrom so thatthe fraction recycled by line 22 is a blend of centrate overflow andunderflow products.

The following table (Table '1) sets out representative distillations ofa coker oil product, hydrocracked naphtha and hydrocracked middledistillate products, and a centrifuge centrate produced by theembodiment of the foregoing process in which a coker oil fractionboiling within a range of from about F. to about 700 F. and containingat least about 20 volume percent of materials boiling below about 400 F.was recycled to the centrifuge feed. In the embodiment, the coalliquefaction product feed rate to the centrifuge was 74 pounds per hourand the coker oil recycle feed rate to the centrifuge was 14.7 poundsper hour (16.6 weight percent of the total feed to the centrifuge). Ashcontent in the feed to the centrifuge was 3.2 weight percent. Centrifugeoverflow liquid was recovered at the rate of 65 pounds per hour andcentrifuge overflow condensate was obtained at the rate of 1 pound perhour. Ash content of the centrifuge overflow liquid was 0.02 weightpercent.

8 runs, the feedto thecentrifuge was modified by inclusion of 10 weightpercent (Run 2) and 20 weight percent (Run As seen from Table I, thecoker oil stream, the hydrocracked naphtha and middle distillatestreams, and the centrifuge overflow, including condensate, all containmaterials boiling below 400 F. The coker oil stream is suitable forrecycle as constituted. The hydrocracked naphtha naphtha distillationrange, and produced as described 3) of a hydrocracked centrate fractionboiling in the above in connection with FIG. 1. Solids in the feeds andash solids in the centrates of Runs 1-3 were measured. The results ofthese runs are set out below in Table II.

TABLE II.-ADDITION OF HYDIiPCRACKED CENTRATE FRACTION (HCF) TO LIQUEACTION PRODUCT (L.P.)

Run number Feedstock L.P. LIA-10% HOF L.P.+20% HCF Feed:

Rate, lb lmln 30. 7 27. 27. 1 Benzene insolubles, wt. percent 1 20. 9421. 23 20. 95 MEK insolubles, wt. percent 6. 14 6. 83 7. 65 Ash, wt.percent 3. 28 3. 34 3. 49 Organic MEK insolubles, wt. percent 2. 86 3.49 4. l6 Quasi-solids, wt. percent 3 14. 8 14. 4 13. 3Overflow/underflow splits:

Quasi-solids 2. 97 4. 58 3. 24 Benzene solubles 3. 02 3. 88 3. 24Centrate:

Rate, lb./min 21. 8 20. 0 19. 6 Specific gravity 1. 0901 1. 0652 1. 0540Viscosity at 400 R, up 3. 7 3. O 2. 9 Ash in centrete, wt. percent 1 0.41 0.16 0. 10 Ash removed from centrate, percent 87. 6 94. 7 96. 6 Ashbalance, percent 103 101 92 Improvement in clarity, percent 61 75. 5

1 Weight percents are based on LP. in feed. 1 Organic MEK insolubles areMEK insolubles less ash. 3 Quasi-solids are benzene insolubles less MEKinsolubles.

fraction may be recycled in whole or blended with the middle distillatesfraction to adjust the middle distillate fraction so that more materialsboiling below 400 F. are included in it. The middle distillate containsabout volume percent of materials boiling below about 400 F. and issuitable for. recycle. The condensate fraction of the centrifugeoverflow may be recycled in whole or blended with the liquid centrifugeoverflow, as described, to produce fractions having at least about 20volume percent boiling below 400 F. (Recycling to condensate will causeit to accumulate and build up to a higher rate than occurred in thecoker oil recycle embodiment just described.)

The following examples will further illustrate the invention. In theexamples, increase in liquid extract clarity is quantified by thedecrease in ash content of solids in the clarified extract. In thisregard, ash content of solids, or reference elsewhere herein to ashsolids, is not the same as the total mineral matter content of thesolids, which also contain sulfur and carbonates, for example.

Example 1 Using a disc-nozzle type centrifuge operating at 400 F.,solids (including ash solids) were separated, in three runs, fromsamples of the liquid extract of a coal liquefaction product resultingfrom heating a slurry of two parts hydrogenated creosote oil and onepart -l00 mesh Illinois #6 coal at 730 F. under 350 p.s.i.g. for aboutminutes. In the first run, the liquefaction product feed to thecentrifuge was unmodified. In the second and third As may be seen byreference to Table I, in Run 1, the ash content of the centrifuge feedwas reduced from 3.28 to 0.41 percent in the centrate, a reduction of87.6 per cent of the ash in the feed. Run 2, in which 10 weight percentof hydrogenated centrate fraction was added to the feed, ash content wasreduced from 3.34 weight percent to 0.16 weight percent in the centrate,an improvement in centrate clarity of 61 percent over that obtained inRun 1. In Run 3, in which 20 weight percent of hydrogenated centratefraction was added to the centrifuge feed, ash content was reduced from3.49 weight percent in the feed to 0.10 weight percent in the centrate,an improvernent in centrate clarity of 75.5 percent over Run 1.

The content of solids in the feed to the centrifuge was determined byusing benzene and methyl ethyl 'ketone (MEK), at room temperature, assolvents to dilute samples of the liquefaction product, at least inequal volumes, and to wash the solids which separated in a laboratoryanalytical centrifuge. The washed solids were pyrolyzed to determine ashcontents. As set forth in Table I, the benzene insoluble solidscomprised solids soluble and insoluble in MEK. The solids which wereinsoluble in MEK had both ash constituents and organic constituents.

The following experiment was undertaken to ascertain the nature of theorganic constituents.

A slurry of two parts by weight of hydrogenated creosote oil to one partby weight of mesh Illinois #6 coal was liquefied at 730 F. and 350p.s.i.g. for about 45 minutes. The liquefaction product was diluted withan equal volume of methyl ethyl ketone, the solids in the dilutedliquefaction product were separated in a laboratory analyticalcentrifuge, and the solids recovered from the centrifuge were washedwith MEK at room temperature. Pyridine was then used as a solvent todilute samples of the MEK washed solids and to wash the solids whichseparated by filtration through Whatman No. 42 fine filter paper.Coulter counter measurements were made on samples of the MEK washedsolids and on samples of the pyridine-washed solids. Solids distributioncurves of the weight percent of solids less than a given size wereplotted against particle sizes for the pyridine washed solids and forthe MEK washed solids. These curves are illustrated in FIG. 2.

The curves in FIG. 2 show that 50 weight percent of the pyridine-washedsolids were less than 6 microns in diameter and 50 weight percent of theMEK washed solids were less than 12 microns in diameter. Thepyridine-washed solids appear to closely approximate the true solidscontent of the solids in the liquefaction extract, i.e., thepyridine-washed solids are composed of the mineral matter andunconvertible organics in the liquefaction extract. The MEK washedsolids, which are larger, are apparently composed of the true solidsplus solid substances which had been dissolved in the liquid extract butwhich had solidified on addition of the MEK to the liquefaction extractto agglomerate the true solids into the larger particles. Thus, theorganic constituents in the MEK insolubles in Run 1 apparently contain aslight amount of converted organic material.

Because identical solvent separation techniques were used in Runs 1-3,valid comparisons can be made between Run 1 and Runs 2 and 3.

Referring to Table II, the solids which were insoluble in benzene butsoluble in methyl ethyl ketone are termed quasi-solids. Benzene solublesare, of course, liquids. The distribution in the centrifuge of thedensity of benzene solubles is given by the ratio of benzene solublesappearing in the overflow to those appearing in the under flow. As Runs1 and 3 indicate, the ratio of quasi-solids in the centrate toquasi-solids, in the under flow is essentially the same as the overflow/underflow ratio for benzene solubles. Hence, the quasi-solids canbe taken to be dissolved in the liquid extract solvent at the 400 F.temperature. (Run 2 had poorer material balances of benzene and MEKinsolubles, not shown in Table 11, and is not illustrative in thisregard.)

As Runs 1 and 3 of Table II further show, on addition of th hydrocrackedcentrate fraction to the coal liquefaction product feed to thecentrifuge the percent of organic MEK insoluble solids in the feedincreased about 1.3 weight percent, the weight percent of quasi-solidsin the feed decreased by about the same amount that the organic MEKinsolubles increased, and the percent of ash stayed fairly consistent.This suggests that the addition of the hydrocracked centrate fraction tothe liquefaction product was instrumental in resolidifying slightly morethan 1 weight percent of the liquid quasi-solids into organic MEKinsoluble solids. Taken with the fact that the clarity of the centraterecovered from the centrifuge increased 75.5 percent on addition of thehydrocracked centrate fraction, the evidence suggests that theresolidification of the quasi-solids was instrumental in increasing thecentrate clarity.

Generally speaking, the solvation power of the liquid extract of theliquefaction product at 400 F. is approximately the same as thesolvation power of pyridine at room temperature, and the solvation powerof the liquid extract plus the hydrocracked centrate fraction at 400 F.is approximately the same as that of MEK at room temperature.Accordingly, the solids distribution curves for pyridine insolublesolids and MEK insoluble solids in FIG. 2 may be taken asrepresentative, respectively, of the sizes of solids in the liquefactionproduct in Run 1 and solids in the liquefaction product containing thehydrocracked centrate fraction in Runs 2 and 3. This indicates that theimproved clarity which accompanied the indicated 10 and solids in theliquefaction product in Rune l and solids in the liqufaction productcontaining the hydrocracked centrate fraction in Rune 2 and 3. Thisindicates that the improved clarity which accompanied the indicatedresolidification of quasi solids in Runs 2 and 3 occurs, apparently,because the resolidification increases the less than 10 micron particlesizes of at least some of the already solid particles in the liquidextract to a size greater than 10 microns, shifting these particles intoa separable size range. The marked nature of the clarity improvementsuggests that the addition of the hydrocracked centrate fractions inRuns 2 and 3 served to agglomerate the smaller than 10 micron particlesin the liquefaction product 7 fed to the centrifuge.

Undoubtedly, in addition to the foregoing, the reduction in specificgravity and viscosity evidenced by Table I also contributed to theincreased resolution in the separation process. The viscosity figures,which were obtained in a Brookfield viscometer, cannot be regarded asquantitatively accurate at the 400 F. operating temperature. However,they can be taken as qualitatively indicative of the relative reductionin viscosity occurring with the addition of the hydrogenated centratefraction.

Example 2, which is set out below, sheds further light on therelationship between specific gravity, ash reduction, and addition of ahydrogenated centrate fraction.

Example 2 In a continuous seven-day run, coal was liquefied andcentrifuged in a solid bowl scroll discharge centrifuge operating atabout 400 F., essentially under atmospheric pressure, while a coker oilfraction derived from a centrifuge underflow (as described in connectionwith FIG. 1) and boiling within the range of from about 100 F. and about700 F. with 20 volume percent thereof boiling below about 400 F. wasadded to the centrifuge feed. In the liquefaction reactor, theconditions which produced the feed to the centrifuge included atemperature of 770 F., a pressure of 350 p.s.i.g., a 7:1 solvent/ coalweight ratio of -100 mesh Illinois #6 coal in hydrogenated creosote oilboiling from about 300 F. to over 1000 F., a coal feed rate to theliquefaction reactor of 8 lbs./hr. and a 1 hour residence time. Exceptas indicated in FIG. 2, the feed rate of centrate coker oil fraction wasabout 2 lbs./ hr., an addition of about 3 weight percent of centratefraction to the centrifuge feed. The centrate rate was about 65 lbs/hr.The results of this seven-day run are displayed in FIG. 3.

FIG. 3 indicates that a monitoring of the specific gravity of thecentrate provides a good measure of the how much more, or less, of coalextract liquid having at least about 20 volume percent of fractionsboiling below about 400 F. needs to be added to the liquefaction productintroduced to a centrifuge separation zone, in a continuous operation,in order to get and keep a desired increase in centrate clarity(reduction in ash content) as the makeup of liquefaction product varies.By controlling the addition of coker oil fraction to keep the specificgravity of the centrate, corrected to 60 F., below about 1.085 in thisexample, it was possible to maintain the ash content below a maximumlevel of about 0.3 weight percent, and, at steady state conditions,below about 0.10 weight percent, as occurred after about 60 hours ofoperation. As illustrated by FIG. 4, at about hours the specific gravityof the centrate, corrected to 60 F., was permitted to creep over the1.085 mark selected for the separation control in the processing of theparticular slurry of this example. Addition of sufficient centratefraction to push the specific gravity back to or below the selected markwas not made, and as illustrated, the maintenance of a uniform centratefraction feed rate without regard to the rising centrate specificgravity permitted the ash content of the centrate to rise.

Because of the relatively higher specific gravity of the clarifiedliquid extract bottoms fraction boiling above 11 about 1000 F. andcontaining ash solids, thejforegoing useof specific gravity measurementsis not believed ap plicable to the bottoms fraction. The bottomsfraction with the ash solids contaminant is surprisingly effective,however, in clarifying the liquid extract, as shown by Example 3.

A solid bowl scroll discharge centrifuge was used to clarify the coalextract of a liquefaction product at a temperature of about 400 F.'Theash content in the clarified coal extract was reduced from approximately3 weight percent to about 0.15 weight percent. As illustrated in FIG. 1,the clarified centrate was then bydrocracked and fractionated. Uponcommencing a recycle to the centrifuge feed of fractionator bottomsboiling above about 1000 F. and containing fine solids which had alreadypassed through'the centrifuge, approximate ly 25 pounds of recyclefraction being added to 81 pounds of the normal feed (about 31 weightpercent), the clarity of the centrate did not deteriorate, as one wouldnormally expect. Instead, the ash content of the centrate surprisinglydecreased to approximately 0.0 weight percent.

Having now fully disclosedwith particularity preferred modes by whichour invention may be carried out various modifications and alterationswithin the spirit and scope of our invention, as claimed, will occur tothose in the art.

We claim:

1. In a process of clarifying a coal liquefaction product containingsolid residues in a solvent enriched by coal extract liquids, by feedingsuch coal liquefaction product into a solids-liquid separation zone inwhich solids size is a parameter of separation for separation of solidresidues from the extract-enriched solvent, separated solids beingdischarged from the separation zone as a high solids content slurry, andthe extract-enriched solvent being discharged from the separation zoneas a clarified extract enriched solvent stream, the improve ment whichcomprises: 7

adding to the feed to said solids-liquids separation zone a coal extractliquid derived from said'coal liquefaction product and containing atleast 20 volume percent of materials boiling below about 400 F., in anamount effective to increase the clarity of the clarifiedextract-enriched solvent stream.

2. The process of claim 1 wherein said amount is in the range from about1 to about 50 weight percent of the resultant stream to said zone. I

3. In a process of clarifying a coal liquefaction product containingsolid residues in a solvent enriched by coal extract liquids, by feedingsuch coal liquefaction product into a solids-liquid separation zone inwhich solids size is a parameter of separation for separation of solidresidues from the extract-enriched solvent, separated solids beingdischarged from the separation zone as a high solids content slurry, andthe extract-enriched solvent being discharged from the separation zoneas a clarified extract enriched solvent stream, me improvement whichcomprises: q,

adding to the feed to said solids-liquids separation zone a coal extractliquid derived from said coal liquefaction product and containing atleast 20 volume percent of materials boiling above about 1000 F., in anamount efiective to increase the clarity of the clarifiedextract-enriched solvent stream.

4. The process of claim 3 wherein said amount is'in the range from about1 to about 50 weight percent of the resultant stream to said zone. v x H5. In a process of clarifying a coal liquefaction product containingsolid residues in a solvent enriched by coal extract liquids, by feedingsuch coal liquefaction product into a solids-liquid separation zone inwhich solids size is a parameter of separation for separation of solidresidues from the extract-enriched solvent, separated ment whichcomprises:

- adding said recycle stream to said liquefaction product adding tosaidcoal liquefaction product a stream selected from:

(a) a fraction of said extract-enriched solvent containing at leastabout 20 volume percent of coal extract materials boiling below 400 F.or'above about 1000 F., Y (b) a fraction of hydrocrackedextract-enriched I solvent containing at least about 20 volume percenthydrocracked coal extract materials boiling below about 400 F. or aboveabout 1000 F., and

(c) a fraction of a coker oil stream containing at least about 20 volumepercent of materials boiling below about 400 F. produced by coking saidslurries of solid residues,

' such stream being added in an amount efiective to resolidifysuflicient of quasi-solid materials of the coal extract to make largerand more easily separable at least a portion of the smaller, ditficultlyseparable solids in the liquefaction product, whereby the clarity of theclarified extract enriched solvent discharged from said solids-liquidsseparation zone is p improved.

6. The process of claim 5 wherein said stream is selected from saidfractions containing at least about 20 volume percent of coal extractmaterials boiling below 400 F., or mixtures thereof, and said' stream isadded to said coal liquefaction product as necessary to keep thespecific gravity of the clarified extract-enriched solvent below aselected value, the range from about 1.08 to about 1.12, at which ashcontent in the clarified extract-enriched solvent exceeds a desiredlimit.

7. The process of claim 5 in which said stream is added in amounts whichconstitute from about 1 to about 50 weight percent of the resultantstream to the solids-liquid separation zone.

.8. The process of claim 5 in which said stream is added in amountseffective to resolidify less than about 5 weight percent of saidquasi-solid materials in said coal extract.

9. The process of claim 5 in which at least a portion of solids of sizesmaller than about 10 microns are made larger and more easily separableon addition of said stream.

10. A method of producing a clarified coal-enriched solvent whichcomprises:

feeding a coal liquefaction product containing solid residues in asolvent enriched with a liquefied coal extract, into a centrifuge, at atemperature from I about 300 F. to about 550 F.,

receiving the clarified overflow and the concentrated solidsunderfiowissuing from the centrifuge, passing the concentrated solids underflowinto a coking zoneoperating to produce distilled products therefrom,

distilling the products from the coking zone and the clarified overflowto recover a recycle stream selected from (a) a fraction boiling withinthe range from about 300 F. to about 700 F. and containing at leastabout 20 volume percent of liquid coal extract materials boiling below400 F., and (b) a fraction boiling above l000 F. which con- V tains ashsolids, and

in amounts which constitute from about 1 to about 50 weight percent ofthe resulting feed to the centrifuge and which are efiective to causethe 13 clarity of the overflow issuing from the centrifuge to improve.

11. The method of claim 10 wherein said recycle stream is said fractionboiling within the range from 300 F. to about 700 F., and wherein saidfraction is added to said liquefaction product as needed to keep thespecific gravity of the clarified overflow below a selected value in therange from about 1.08 to about 1.12 at which ash content in theclarified extract-enriched solvent exceeds a desired limit.

12. The process of claim 10 wherein said recycle stream is added to saidliquefaction product so as to constitute less than about 20 volumepercent of the resulting feed to the centrifuge.

References Cited UNITED STATES PATENTS DELBERT E. GANTZ, PrimaryExaminer 10 J. W. HELLWEGE, Assistant Examiner US. Cl. X.R.

